1. Field
Certain aspects of the present disclosure generally relate to wireless communications and, more particularly, to a method of designing Media Access Control (MAC) scheduler for uplink communication in high rate wireless data systems, with specific application to Long Term Evolution (LTE) systems.
2. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems, Long Term Evolution Advanced (LTE-A) systems, and Orthogonal Frequency Division Multiple Access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-input single-output, multiple-input single-output or a multiple-input multiple-output (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system supports a time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point to extract transmit beamforming gain on the forward link when multiple antennas are available at the access point.
A scheduler can be designed within a base station to allocate uplink communication resources to a mobile terminal in a sector served by the base station. The allocation may be based on channel conditions, an amount of interference the mobile terminal causes to other neighboring sectors, quality of service (QoS) constraints of a traffic that the mobile terminal requests to transmit, and delayed information about a buffer state at the mobile terminal made available to the serving base station. In particular, the scheduler may need to provide a fair rate allocation to all served mobile terminals. For example, mobile terminals at a cell edge should be assigned to a relatively lower transmission rate, but they also should not be starved all the time.
The 3rd generation (3G) and 4th generation (4G) wireless cellular systems can typically serve a large number of flows with diverse requirements. Also, in the 4G wireless systems, scheduling decisions may be made approximately every millisecond, and spectral resources may be distributed among different mobile terminals with a high granularity. Hence, it may be desirable that computing of scheduling decisions is efficient, in addition to ensuring fairness among the mobile terminals.
Prior art approaches to the above issues rely on one or both of the following techniques to reduce the computational complexity of scheduling algorithm.
In one aspect, the reduction of scheduling complexity may be based on a strict prioritization among flow classes. Classes of flows (where a class may depend on negotiated QoS) may be scheduled in the order of importance, i.e., more important classes may have a higher scheduling priority. Then, during each scheduling instance transmission packets associated with higher priority flows may be served before transmission packets associated with lower priority flows. This may be performed irrespective of channel and interference conditions associated with mobile terminals to which the flows belong. However, this approach may be highly suboptimal because it may not exploit multiuser diversity nor it may ensure fairness among the mobile terminals.
In another aspect, the reduction of scheduling complexity may be based on a greedy scheduling in an order of current priority. Priority of all flows may be computed based on a current buffer state and a channel state associated with a served mobile terminal. Then, the flows may be served in decreasing order of priority. Once a flow is chosen, enough bandwidth may be allocated either to empty the buffer or to use a maximum power at the mobile terminal, which may be limited by the device transmit power or intra/inter-cell interference constraints. However, this approach may be also highly suboptimal because all transmission packets in the buffer may usually have different priorities. For example, a flow may obtain a high priority only because a fraction of packets belonging to that flow may have the high priority.